Adhesion of nickel to chromium



United States Pat nt" 3,002,899 ADHESION OF NICKEL T CHROMIUM Walter E. Reid, Jr., Washington, D.C., assignor to the United States of America as represented by the Secretary of the Navy No Drawing. Filed July 31, 1959, Ser. No. 830,959

3 Claims. (Cl. 204-32) (Granted under Title 35, U.S. Code (1952), see. 266) The invention described herein may be manufactured and used by or for the Government of the United States of America for governmental purposes without the payment of any royalties thereon or therefor.

This invention relates to a method of metal electroplating, -and more particularly to a method of obtaining good adhesion of nickel to chromium at elevated temperatures.

Most procedures used to plate nickel on chromium yield good adhesion of the nickel to the chromium in the as plated state. However, upon heating the coating it has been found that blisters form at temperatures as low as 306 C. Generally speaking heating the coating to between 300" C. and 1200 C. for about an hour has shown that the conventional or prior art methods of plating nickel on chromium fail as a result of the formation of blisters, poor adhesion of gross magnitude and/or edge separation.

As a result of investigation into the reasons for the failure of the coatings to adhere properly upon exposure to high temperatures it was determined that two factors can affect the adhesion of the deposits: (1) inadequate cleansingof the chromium surface, and (2) alloying of the chromium. Although the second factor appears to be inherent in the Cr-Ni coating system the present invention is directed toward eliminating the first factor afiecting the adhesion of nickel to chromium.

This invention is of prime import since exhaustive research efforts have been directed toward seeking a suitable coating for molybdenum, for example, to prevent its rapid oxidation at elevated temperatures. Theoretically the employment of a combination of chromium and nickel has been the most attractive solution to the problem. However, the nature of the base is of no import in this invention and the present effort is directed toward the chromium layer-nickel layer combination.

Chromium that has been exposed to oxygen becomes passive due to the formation of an oxide film. Two methods have been used to activate the surface before the plating step. These methods are (1) immersion of the chromium plated specimen in 1:1 hydrochloric acid, and (2) etching the chromium anodically in a glacial acetic acid solution. The use of hydrochloric acid has givengood results, but it is difiicult to consistently obtain coatings which remain free of blisters after heating to 800 C. By the use of the procedure of anodic etching of the chromium prior to plating with nickel, deposits yielding good adhesion and freedom from blisters when exposed to temperatures as high as 850 C. for as long as 24 hours have been consistently obtained.

It is therefore a general object of the present invention to provide a new procedure for activating a chromium surface prior to plating with nickel whereby deposits exhibiting good adhesion and freedom from blisters when exposed to elevated temperatures are obtained.

Other objects and advantages of the present invention will be readily appreciated as the same becomes better Ice understood by reference to the following detailed description.

According to the present process for etching a chromium surface the specimen (having a chromium surface) is made the anode in an electrolytic solution composed of glacial acetic acid containing 10 to 15 percent by volume of concentrated sulfuric acid (commercial C-P) to increase the conductivity. An etching time of about 5 minutes (until the specimen becomes coated with a dark brown film) at a current density of 3 to 7 amp/dm. is employed. The cathode used in the etching process is of an inert material such as carbon. After the etching step the specimen is rinsed in water to remove the dark brown film. Next the specimen is placed in a Woods nickel strike bath with the current on for from about 2 to 4 minutes at a current density of 20 to 35 amp./dm. The composition of a gallon of Woods nickel strike bath is nickel chloride (32 02.), hydrochloric acid (16 oz.) and water. In order to avoid any reaction with the HCl the current is turned on before the specimen, which is the cathode, is inserted into the bath whereby electrodeposition begins immediately. Then the specimen is plated in the conventional manner in the Watts nickel plating bath.

In evaluating the above procedure these variables were examined: 1) the etching time and (2) the time of exposure to air between the water rinse and the insertion of the specimen into the Woods nickel strike bath.

The etching time was varied from one to five minutes but did not appear to affect the adhesion of the coatings. In general, an etching time of 5 minutes at 4 amp./dm. is preferred.

The exposure time between the water rinse and insertion of the specimen into the Woods nickel strike bath with current on was varied for intervals up to five minutes. The adhesion of the coatings after exposure to elevated temperatures did not appear to be affected even though most of the specimens became dry during the waiting period.

Occasionally, at high current densities, gas evolution was observed from the chromium during the etching process.

Further, the present plating procedure produces coatings which have such good adhesion that this adhesion does not appear to be grossly afiected by heating the specimens for 24 hours at 850 C. Also the overall adhesion is unaffected by submitting the specimens to a 24 hour cycling test in which the specimens are heated to 850 0.; held at that temperature for 10 minutes; cooled to room temperature; held at that temperature for 10 minutes and then the cycle is repeated.

Obviously many modifications and variations of the present invention are possible in the light of the above teachings. It is therefore to be understood that within the scope of the appended claims the invention may be practiced otherwise than as specifically described.

What is claimed is:

1. A process for preparing a chromium surface to yield improved adhesion at elevated temperatures with a nickel coating to be deposited thereon which comprises the steps of anodically etching the chromium surface at a current density of about 3 to about 7 amp/rim. in an electrolyte solution composed of glacial acetic acid containing about 10 to about 15 percent by volume of concentrated sulfuric acid until said surface becomes coated with a dark brown gfiim, rinsing.- said surface with water and-giving said surface a nickel strike in a Woods nickel strike bath.

2. The process described in claim 1 wherein the period of time for the anodical etching is about 5 minutes.

3. The process described in claim 1 wherein the anodically etched chromium surface is placed in the nickel strike bath for from about 2 to about 4 minutes at 21 current density from about 20 to about 35 ampjdmfi.

- *References-Citedinthefile. otthis; patent UNITED STATES PATENTS 1,787,672 Davenport Jan. 6, 1931 2,336,846 Clark Dec. 14, 1943 2,421,316 Carson et a1. May 27, 1947 2,491,126 McGill Dec. 13, 1949 2,570,174 Weber Oct. 2, 1951 

1. A PROCESS FOR PREPARING A CHROMIUM SURFACE TO YIELD IMPROVED ADHESION AT ELEVATED TEMPERATURES WITH A NICKEL COATING TO BE DISPOSITED THEREON WHICH COMRPISES THE STEPS OF ANODICALLY ETCHING THE CHROMIUM SURFACE AT A CURRENT DENSITY OF ABOUT 3 TO ABOUT 7 AMP./DM.2 IN AN ELECTROLYTE SOLUTION COMPOSED OF GLACIAL ACETIC ACID CONTAINING ABOUT 10 TO ABOUT 15 PERCENT BY VOLUME OF CONCENTRATED SULFURIC ACID UNTIL SAID SURFACE BECOMES COATED WITH A DARK BROWN FILM, RISING SAID SURFACE WITH WATER AND GIVING SAID SURFACE A NICKEL STRIKE IN A WOOD''S NICKEL STRIKE BATH. 